The present invention relates generally to methods of manufacturing micro-electromechanical devices and, more particularly, to methods for manufacturing thermally actuated manufacturing liquid control devices such as the type used in liquid drop emitters, ink jet printheads and microfluidic valves.
Micro-electro mechanical systems (MEMS) are a relatively recent development. Such MEMS are being used as alternatives to conventional electro-mechanical devices as actuators, valves, and positioners. Micro-electromechanical devices are potentially low cost, due to use of microelectronic fabrication techniques. Novel applications are also being discovered due to the small size scale of MEMS devices.
Many potential applications of MEMS technology utilize thermal actuation to provide the motion needed in such devices. For example, many actuators, valves and positioners use thermal actuators for movement. In some applications the movement required is pulsed. For example, rapid displacement from a first position to a second, followed by restoration of the actuator to the first position, might be used to generate pressure pulses in a fluid or to open or close a fluid flow valve. Drop-on-demand liquid drop emitters use discrete pressure pulses to eject discrete amounts of liquid from a nozzle.
Drop-on-demand (DOD) liquid emission devices have been known as ink printing devices in ink jet printing systems for many years. Early devices were based on piezoelectric actuators such as are disclosed by Kyser et al., in U.S. Pat. No. 3,946,398 and Stemme in U.S. Pat. No. 3,747,120. A currently popular form of inkjet printing, thermal ink jet (or xe2x80x9cbubble jetxe2x80x9d), uses electrically resistive heaters to generate vapor bubbles which cause drop emission, as is discussed by Hara et al., in U.S. Pat. No. 4,296,421.
Electrically resistive heater actuators have manufacturing cost advantages over piezoelectric actuators because they can be fabricated using well developed microelectronic processes. On the other hand, the thermal ink jet drop ejection mechanism requires the ink to have a vaporizable component, and locally raises ink temperatures well above the boiling point of this component. This temperature exposure places severe limits on the formulation of inks and other liquids that may be reliably emitted by thermal ink jet devices.
The availability, cost, and technical performance improvements that have been realized by ink jet device suppliers have also engendered interest in the devices for other applications requiring micro-metering of liquids. These new applications include dispensing specialized chemicals for micro-analytic chemistry as disclosed by Pease et al., in U.S. Pat. No. 5,599,695; dispensing coating materials for electronic device manufacturing as disclosed by Naka et al., in U.S. Pat. No. 5,902,648, and for dispensing microdrops for medical inhalation therapy as disclosed by Psaros et al., in U.S. Patent 5,771,882. Devices and methods capable of emitting, on demand, micron-sized drops of a broad range of liquids are needed for highest quality image printing, but also for emerging applications where liquid dispensing requires mono-dispersion of ultra small drops, accurate placement and timing, and minute increments.
A low cost approach to micro drop emission is needed which can be used with a broad range of liquid formulations. Methods of manufacture are needed which utilize the cost advantages of microelectronic fabrication to form mechanical actuators which can usefully perform in contact with a variety of working fluid chemistries and formulations.
A DOD ink jet device which uses a thermo-mechanical actuator was disclosed by T. Kitahara in JP 2,030,543, filed Jul. 21, 1988. The actuator is configured as a bi-layer cantilever moveable within an ink jet chamber. The beam is heated by a resistor causing it to bend due to a mismatch in thermal expansion of the layers. The free end of the beam moves to pressurize the ink at the nozzle causing drop emission.
A second configuration of a DOD ink jet device which uses a thermo-mechanical actuator was disclosed by Matoba, et al. in U.S. Pat. No. 5,684,519. The actuator is formed as a thin beam constructed of a single electroresistive material located in an ink chamber opposite an ink ejection nozzle. The beam buckles due to compressive thermo-mechanical forces when current is passed through the beam. The beam is pre-bent into a shape bowing towards the nozzle during fabrication so that the thermo-mechanical buckling always occurs in the direction of the pre-bending.
A microvalve device which uses a thermo-mechanical actuator was disclosed by Wood, et al., in U.S. Pat. No. 5,909,078. The actuator is configured as an arched beam which extends between spaced apart supports on a microelectronic substrate. The arched beam expands when heated either from an external source or internally by passing current through an electrically resistive layer in the beam. A coupler mechanically couples the arched beam to a valve plate to open and close a fluid microvalve.
Thermo-mechanical actuators having either cantilevered members with free ends, or anchored members with at least two free opposing edges to allow movement, are useful in fluid control devices such as liquid drop emitters or microvalves because they provide substantial mechanical displacement for a given input of thermal energy. However, configurations which have moveable edges are especially susceptible to damage and failure at the exposed actuator edges from chemical interactions between the materials of the actuator and components or impurities in the working fluid used.
The thermal expansion gradients which cause the desired movement of the actuator member may be generated by temperature gradients, by materials changes, layers, which expand differently at elevated temperatures, or by a combination of both effects during a thermal cycle. It is advantageous for pulsed thermal actuators to be able to establish and dissipate thermal expansion gradients quickly, so that the actuator can be cycled at a high rate. The thickness and thermal conductivity of each actuator layer, and passive heat conduction pathways are very important considerations in the design and fabrication of an energy efficient device.
Methods of manufacturing thermal actuators for liquid control devices are needed which successfully accommodate requirements for low cost, mechanical performance, thermal efficiency, and chemical reliability in the face of chemically active working fluids.
Liquid drop emitters require a highly accurate nozzle opening which communicates to a liquid chamber in which the moveable thermal actuator generates drop emission pressures. In many applications, such as ink jet printheads, large numbers of drop emitters, jets, are fabricated in spatially dense arrays in order to achieve high printing speeds and image quality. Such arrays of jets are only useful if the individual nozzles are extremely uniform in their geometrical parameters, especially shape, bore length, and surface planarity. In addition, maintenance of drop emission performance during use may require periodic wiping of the nozzle face area. The strength and topography of the liquid chamber and nozzle wall are important contributors to the design of a reliable ink printhead and printhead maintenance subsystem combination.
Methods of manufacturing liquid control devices are needed which integrate strong chamber structures in which the actuator moves freely against the working fluid. In addition, methods of manufacturing liquid chamber structures which integrate highly accurate and uniform liquid exit nozzles are needed for thermally actuated liquid drop emitters, especially ink jet printheads.
Recently, disclosures of thermo-mechanical DOD ink jet configurations and methods of manufacture have been made by K. Silverbrook in U.S. Pat. Nos. 6,067,797; 6,087,638; 6,180,427; 6,217,153; and 6,228,668 (hereinafter, xe2x80x9cthe Silverbrook patentsxe2x80x9d). A variety of microelectronic materials, processes and process sequences are described. However, the disclosed fabrication methods do not address the need to form thermal actuators which combine thermal efficiency and protection of the actuator materials from chemical interactions. The disclosed manufacturing methods and materials do not allow the use of high temperature deposition processes for layers which need to have contact with the ink jet ink. Also, the disclosed manufacturing methods do not provide for a liquid chamber structure which is suited for the formation of dense arrays of jets having highly uniform nozzles. Further, the disclosed manufacturing methods result in drop emitter devices having nozzle faces with topographical features that may trap debris and be difficult to maintain via wiping methods.
Methods of manufacturing thermally actuated liquid control devices, especially liquid drop emitters, are needed which combine the features of low cost microelectronic fabrication processes and materials, thermally efficient design, wet chemical passivation, and mechanically robust liquid chamber structures with accurately formed, maintainable, nozzles.
It is therefore an object of the present invention to provide a method of manufacturing a thermal actuator having free edges for a liquid control device which is thermally efficient and protected from chemical interactions with the working liquid.
It is also an object of the present invention to provide method of manufacturing a movement volume, especially a liquid chamber, which can be integrally formed with a thermal actuator.
It is further an object of the present invention to provide a method of manufacturing a strong liquid chamber for a liquid drop emitter, especially an ink jet printhead, which has accurately formed nozzle openings and can be integrally formed with a thermal actuator.
The foregoing and numerous other features, objects and advantages of the present invention will become readily apparent upon a review of the detailed description, claims and drawings set forth herein. These features, objects and advantages are accomplished by a method for manufacturing a thermal actuator for a micro-electromechanical device comprising the steps of forming a bottom layer of a bottom material on a substrate having a flat surface and composed of a substrate material, and removing the bottom material in a bottom layer pattern wherein a moveable area located between opposing free edges remains on the substrate. A deflector layer of a deflector material is formed over the bottom layer and patterned so that the deflector material does not overlap the free edges of the bottom layer material. A top layer of a top material is formed over the deflector layer, the bottom layer, and the substrate and patterned so that the top material overlaps the deflector layer material but does not completely overlap the substrate material in the free edge area. A layer of a sacrificial material is conformed over the top, deflector, bottom layers and substrate in sufficient thickness to result in a planar sacrificial layer surface parallel to the flat surface. The sacrificial material is patterned so that sacrificial material remains in movement areas and adjacent free edge areas. A structure layer of a structure material is formed over the sacrificial layer and patterned to have openings which expose the sacrificial material in movement areas. The substrate material beneath the moveable area is removed so that the free edges of the bottom layer are released from the substrate and the exposed sacrificial material is removed from the movement areas and free edge areas thereby creating a movement volume for the thermal actuator. High temperature microelectronic fabrication processes may be used for forming the bottom, deflector and top layer materials. The openings in the structure material may serve as nozzles for a liquid drop emitter or as inlet or outlet ports for a microvalve.
The present invention is particularly useful to construct liquid drop emitters used as printheads for DOD ink jet printing. In some preferred embodiments of the inventions, the deflector layer of the thermal actuator may be formed with an electrically resistive material, especially titanium aluminide, the bottom layer may be formed by oxidation of the substrate, and the sacrificial material may be non-photoimageable polyimide.